Mechanism of coordinated synthesis of the antagonistic regulatory proteins NifL and NifA of Klebsiella pneumoniae

Functional and Physiological Characterization of Tyrosine Decarboxylases from Olea europaea L. Involved in the Synthesis of the Main Phenolics in Olive Fruit and Virgin Olive Oil

The phenolic composition of virgin olive oil (VOO) primarily depends on the phenolic content of the olive fruit. The purpose of this work was to characterize the first metabolic step in the synthesis of tyrosol (Ty) and hydroxytyrosol (HTy), whose derivatives are by far the predominant phenolics in both olive fruit and VOO. To this end, two genes encoding tyrosine/DOPA decarboxylase enzymes, OeTDC1 and OeTDC2, have been identified and functionally and physiologically characterized. Both olive TDC proteins exclusively accept aromatic amino acids with phenolic side chains, such as tyrosine and 3,4-dihydroxyphenylalanine (DOPA), as substrates to produce tyramine and dopamine, respectively. These proteins exhibited a higher affinity for DOPA than for tyrosine, and the catalytic efficiency of both proteins was greater when DOPA was used as a substrate. Both olive TDC genes showed a fairly similar expression profile during olive fruit ontogeny, with OeTDC1 consistently expressed at higher levels than OeTDC2. Expression was particularly intense during the first few weeks after fruit set, coinciding with the active accumulation of Ty and HTy derivatives. The data suggest that both olive TDCs are responsible for the initial step in the synthesis of the most important phenolics, both quantitatively and functionally, in VOO.

The bacterial effector HopZ1a acetylates MKK7 to suppress plant immunity

The Pseudomonas syringae type III secretion system translocates effector proteins into the host cell cytosol to suppress plant basal immunity. Effector HopZ1a suppresses local and systemic immunity triggered by pathogen-associated molecular patterns (PAMPs) and effectors, through target acetylation. HopZ1a has been shown to target several plant proteins, but none fully substantiates HopZ1a-associated immune suppression. Here, we investigate Arabidopsis thaliana mitogen-activated protein kinase kinases (MKKs) as potential targets, focusing on AtMKK7, a positive regulator of local and systemic immunity. We analyse HopZ1a interference with AtMKK7 by translocation of HopZ1a from bacteria inoculated into Arabidopsis expressing MKK7 from an inducible promoter. Reciprocal phenotypes are analysed on plants expressing a construct quenching MKK7 native expression. We analyse HopZ1a-MKK7 interaction by three independent methods, and the relevance of acetylation by in vitro kinase and in planta functional assays. We demonstrate the AtMKK7 contribution to immune signalling showing MKK7-dependent flg22-induced reactive oxygen species (ROS) burst, MAP kinas (MAPK) activation and callose deposition, plus AvrRpt2-triggered MKK7-dependent signalling. Furthermore, we demonstrate HopZ1a suppression of all MKK7-dependent responses, HopZ1a-MKK7 interaction in planta and HopZ1a acetylation of MKK7 with a lysine required for full kinase activity. We demonstrate that HopZ1a targets AtMKK7 to suppress local and systemic plant immunity.

Complex Interplay between FleQ, Cyclic Diguanylate and Multiple σ Factors Coordinately Regulates Flagellar Motility and Biofilm Development in Pseudomonas putida

Most bacteria alternate between a free living planktonic lifestyle and the formation of structured surface-associated communities named biofilms. The transition between these two lifestyles requires a precise and timely regulation of the factors involved in each of the stages that has been likened to a developmental process. Here we characterize the involvement of the transcriptional regulator FleQ and the second messenger cyclic diguanylate in the coordinate regulation of multiple functions related to motility and surface colonization in Pseudomonas putida. Disruption of fleQ caused strong defects in flagellar motility, biofilm formation and surface attachment, and the ability of this mutation to suppress multiple biofilm-related phenotypes associated to cyclic diguanylate overproduction suggests that FleQ mediates cyclic diguanylate signaling critical to biofilm growth. We have constructed a library containing 94 promoters potentially involved in motility and biofilm development fused to gfp and lacZ, screened this library for FleQ and cyclic diguanylate regulation, and assessed the involvement of alternative σ factors σ^N and FliA in the transcription of FleQ-regulated promoters. Our results suggest a dual mode of action for FleQ. Low cyclic diguanylate levels favor FleQ interaction with σ^N-dependent promoters to activate the flagellar cascade, encompassing the flagellar cluster and additional genes involved in cyclic diguanylate metabolism, signal transduction and gene regulation. On the other hand, characterization of the FleQ-regulated σ^N- and FliA-independent PlapA and PbcsD promoters revealed two disparate regulatory mechanisms leading to a similar outcome: the synthesis of biofilm matrix components in response to increased cyclic diguanylate levels.

Mechanism of Antiactivation at the Pseudomonas sp. Strain ADP σN-Dependent PatzT Promoter

PatzT is an internal promoter of the atzRSTUVW operon that directs the synthesis of AtzT, AtzU, AtzV, and AtzW, components of an ABC-type cyanuric acid transport system. PatzT is σ(N) dependent, activated by the general nitrogen control regulator NtrC with the assistance of protein integration host factor (IHF), and repressed by the LysR-type transcriptional regulator (LTTR) AtzR. We have used a variety of in vivo and in vitro gene expression and protein-DNA interaction assays to assess the mechanisms underlying AtzR-dependent repression of PatzT Here, we show that repression only occurs when AtzR and NtrC interact simultaneously with the PatzT promoter region, indicating that AtzR acts as an antiactivator to antagonize activation by NtrC. Furthermore, repression requires precise rotational orientation of the AtzR and NtrC binding sites, strongly suggesting protein-protein interaction between the two proteins on the promoter region. Further exploration of the antiactivation mechanism showed that although AtzR-dependent repression occurs prior to open complex formation, AtzR does not alter the oligomerization state of NtrC or inhibit NtrC ATPase activity when bound to the PatzT promoter region. Taken together, these results strongly suggest that PatzT-bound AtzR interacts with NtrC to prevent the coupling of NtrC-mediated ATP hydrolysis with the remodeling of the interactions between E-σ(N) and PatzT that lead to open complex formation.

IMPORTANCE

Here, we describe a unique mechanism by which the regulatory protein AtzR prevents the activation of the σ(N)-dependent promoter PatzT Promoters of this family are always positively regulated, but there are a few examples of overlapping negative regulation. The mechanism described here is highly unconventional and involves an interaction between the repressor and activator proteins to prevent the action of the repressor protein on the RNA polymerase-promoter complex.

Redox proteins of hydroxylating bacterial dioxygenases establish a regulatory cascade that prevents gratuitous induction of tetralin biodegradation genes

Bacterial dioxygenase systems are multicomponent enzymes that catalyze the initial degradation of many environmentally hazardous compounds. In Sphingopyxis granuli strain TFA tetralin dioxygenase hydroxylates tetralin, an organic contaminant. It consists of a ferredoxin reductase (ThnA4), a ferredoxin (ThnA3) and a oxygenase (ThnA1/ThnA2), forming a NAD(P)H–ThnA4–ThnA3–ThnA1/ThnA2 electron transport chain. ThnA3 has also a regulatory function since it prevents expression of tetralin degradation genes (thn) in the presence of non-metabolizable substrates of the catabolic pathway. This role is of physiological relevance since avoids gratuitous and wasteful production of catabolic enzymes. Our hypothesis for thn regulation implies that ThnA3 exerts its action by diverting electrons towards the regulator ThnY, an iron-sulfur flavoprotein that together with the transcriptional activator ThnR is necessary for thn gene expression. Here we analyze electron transfer among ThnA4, ThnA3 and ThnY by using stopped-flow spectrophotometry and determination of midpoint reduction potentials. Our results indicate that when accumulated in its reduced form ThnA3 is able to fully reduce ThnY. In addition, we have reproduced in vitro the regulatory circuit in the proposed physiological direction, NAD(P)H–ThnA4–ThnA3–ThnY. ThnA3 represents an unprecedented way of communication between a catabolic pathway and its regulatory system to prevent gratuitous induction.

An A-tract at the AtzR binding site assists DNA binding, inducer-dependent repositioning and transcriptional activation of the PatzDEF promoter

The LysR-type regulator AtzR activates the Pseudomonas sp. ADP atzDEF operon in response to nitrogen limitation and cyanuric acid. Activation involves repositioning of the AtzR tetramer on the PatzDEF promoter and relaxation of an AtzR-induced DNA bend. Here we examine the in vivo and in vitro contribution of an A5 -tract present at the PatzDEF promoter region to AtzR binding and transcriptional activation. Substitution of the A-tract for the sequence ACTCA prevented PatzDEF activation and high-affinity AtzR binding, impaired AtzR contacts with the activator binding site and shifted the position of the AtzR-induced DNA bend. Analysis of a collection of mutants bearing different alterations in the A-tract sequence showed that the extent of AtzR-dependent activation does not correlate with the magnitude or orientation of the spontaneous DNA bend generated at this site. Our results support the notion that indirect readout of the A-tract-associated narrow minor groove is essential for the AtzR-DNA complex to achieve a conformation competent for activation of the PatzDEF promoter. Conservation of this motif in several binding sites of LysR-type regulators suggests that this mechanism may be shared by other proteins in this family.

Transcriptional activation of the CrcZ and CrcY regulatory RNAs by the CbrB response regulator in Pseudomonas putida

The CbrAB two-component system has been described as a high-ranked element in the regulatory hierarchy of Pseudomonas putida that controls a variety of metabolic and behavioural traits required for adaptation to changing environmental conditions. We show that the response regulatory protein CbrB, an activator of σ(N) -dependent promoters, directly controls the expression of the small RNAs CrcZ and CrcY in P. putida. These two RNAs sequester the protein Crc, which is a translational repressor of multiple pathways linked to carbon catabolite repression. We characterized the in vivo and in vitro activation by CbrB at both crcZ and crcY promoters, and identified new DNA sequences where the protein binds. IHF, a co-activator at many σ(N) -dependent promoters, also binds to the promoter regions and contributes to the activation of the sRNAs. CbrB phosphorylation is necessary at physiological activation conditions, but a higher dose of the protein allows in vitro transcriptional activation in its non-phosphorylated form. We also show there is some production of CrcY coming from an upstream promoter independent of CbrB. Thus, CbrAB constitute a global signal transduction pathway integrated in a higher regulatory network that also controls catabolite repression through the expression of the two regulatory RNAs CrcZ and CrcY.

Transcriptional organization and regulatory elements of a Pseudomonas sp. strain ADP operon encoding a LysR-type regulator and a putative solute transport system

The atzS-atzT-atzU-atzV-atzW gene cluster of the Pseudomonas sp. strain ADP atrazine-degradative plasmid pADP-1, which carries genes for an outer membrane protein and the components of a putative ABC-type solute transporter, is located downstream from atzR, which encodes the LysR-type transcriptional regulator of the cyanuric acid-degradative operon atzDEF. Here we describe the transcriptional organization of these genes. Our results show that all six genes are cotranscribed from the PatzR promoter to form the atzRSTUVW operon. A second, stronger promoter, PatzT, is found within atzS and directs transcription of the four distal genes. PatzT is σ(N) dependent, activated by NtrC in response to nitrogen limitation with the aid of IHF, and repressed by AtzR. A combination of in vivo mutational analysis and primer extension allowed us to locate the PatzT promoter and map the transcriptional start site. Similarly, we used deletion and point mutation analyses, along with in vivo expression studies and in vitro binding assays, to locate the NtrC, IHF, and AtzR binding sites and address their functionality. Our results suggest a regulatory model in which NtrC activates PatzT transcription via DNA looping, while AtzR acts as an antiactivator that diminishes expression by interfering with the activation process.

Molecular and biochemical characterization of the tetralin degradation pathway in Rhodococcus sp. strain TFB

The tetralin biodegradation pathway in Rhodococcus sp. strain TFB, a Gram‐positive bacterium resistant to genetic manipulation, was characterized using a proteomic approach. Relative protein expression in cell free extracts from tetralin‐ and glucose‐grown cells was compared using the 2D‐DIGE technique. Identification of proteins specifically expressed in tetralin‐grown cells was used to characterize a complete set of genes involved in tetralin degradation by reverse genetics. We propose a tetralin degradation pathway analogous to that described for Sphingomonas macrogolitabida strain TFA. TFB thn genes are organized into three operons; two contain all of the structural genes and are transcribed in the same direction, while the third operon, thnST, is transcribed in the opposite direction and encodes a two‐component regulatory system, whose transcription is higher in tetralin‐grown cells. In addition to tetralin induction, TFB thn structural genes are subject to glucose repression. Primer extension assays and translational thnA1::gfp and thnS::gfp fusions were used to characterize putative promoter regions. A mutational analysis of the thnA1 promoter region allowed us to define nucleotides within the cis regulatory elements that are important for the control of thn gene expression.

Regulation of glutamate dehydrogenase expression in Pseudomonas putida results from its direct repression by NtrC under nitrogen-limiting conditions

Nitrogen-regulated genes in enterobacteria are positively controlled by the transcriptional activator of σ(N) -dependent promoters NtrC, either directly or indirectly, through the dual regulator Nac. Similar to enterobacteria, gdhA encoding glutamate dehydrogenase from Pseudomonas putida is one of the few genes that is induced by excess nitrogen. In P. putida, the binding of NtrC to the gdhA promoter region and in vitro transcription suggest that, unlike its enterobacterial homologue that is repressed by Nac, gdhA is directly repressed by NtrC. Footprinting analyses demonstrated that NtrC binds to four distinct sites in the gdhA promoter. NtrC dimers bind cooperatively, and those bound closer to the promoter interact with the dimers bound further upstream, thus producing a proposed repressor loop in the DNA. The formation of the higher-order complex and the repressor loop appears to be important for repression but not absolutely essential. Both the phosphorylated and the non-phosphorylated forms of NtrC efficiently repressed gdhA transcription in vitro and in vivo. Therefore, NtrC repression of gdhA under nitrogen-limiting conditions does not depend on the phosphorylation of the regulator; rather, it relies on an increase in the repressor concentration under these conditions.

ThnY is a ferredoxin reductase-like iron-sulfur flavoprotein that has evolved to function as a regulator of tetralin biodegradation gene expression

Previous genetic studies in Sphingomonas macrogolitabida strain TFA have established that expression of genes involved in tetralin biodegradation (thn genes) requires the function of the LysR type activator ThnR and also ThnY. Sequence comparison indicated that ThnY is homologous to bacterial oxygenase-coupled NAD(P)H-dependent ferredoxin reductases. However, ThnY showed substitutions in highly conserved positions of the pyridine nucleotide binding domain of these ferredoxin reductases. ThnY expression is co-regulated with all other genes required for tetralin biodegradation, and presumably thnY is part of the thnCA3A4RY operon. ThnY has been purified, and its biochemical and functional properties were characterized. ThnY was found to be a monomeric orange-brown iron-sulfur flavoprotein (estimated mass of 37,000 Da) containing one non-covalently attached flavin adenine dinucleotide and one plant type ferredoxin 2Fe-2S cluster. It can be efficiently reduced by dithionite, but reduction by pyridine nucleotides was very poor. Consistently, ThnY-dependent reduction of cytochrome c, ferricyanide, or 2,6-dichlorophenolindophenol using NAD(P)H as the electron donor was undetectable or very weak. The addition of ThnY to electrophoretic mobility shift assays containing ThnR and a probe bearing two thn divergent promoters resulted in a 3-fold increase in protein-DNA complex formation affinity, which indicates that ThnY directly promotes thn transcription activation by ThnR.

Complex interplay between the LysR-type regulator AtzR and its binding site mediates atzDEF activation in response to two distinct signals

AtzR is a LysR-type regulator responsible for activation of the cyanuric acid utilization operon atzDEF. AtzR binds the PatzDEF promoter region at a strong recognition element, designated the repressor binding site, and a weaker binding determinant, the activator binding site (ABS). AtzR activates transcription in response to two dissimilar signals, nitrogen limitation and cyanuric acid. In the present work we analyse the structure and function of the cis-acting elements involved in AtzR activation of atzDEF. Hydroxyl radical footprinting assays revealed that the ABS is composed of three functional subsites spaced at one helix-turn intervals. Two modes of interaction with the ABS are detected in vitro: AtzR binds at the ABS-2 and ABS-3 subsites in the absence of inducer, and relocates to interact with the ABS-1 and ABS-2 subsites in the presence of cyanuric acid. In vivo mutational analysis indicates that ABS-1 and ABS-2 are required for full PatzDEF activation in all conditions. In contrast, ABS-3 acts as a 'subunit trap' that hinders productive AtzR interactions with ABS-1 and ABS-2. Our results strongly suggest an activation model in which cyanuric acid and nitrogen limitation cooperate to reposition AtzR from an inactive, ABS-3 bound configuration to an active, ABS-1- and ABS-2-bound configuration.

Co-ordinated regulation of two divergent promoters through higher-order complex formation by the LysR-type regulator ThnR

The genes required for tetralin biodegradation by Sphingomonas macrogolitabida strain TFA are clustered in two divergent and closely linked operons. ThnR, a LysR-type regulator, activates transcription from each operon in response to tetralin. The regulatory thnR gene is co-transcribed with the catabolic genes thnC, thnA3 and thnA4, resulting in positive autoregulation. ThnR binds with different affinity to two primary binding sites, designated B and C, in the intervening region between the two operons and makes additional contact with secondary sites that extend towards the promoters. In addition, ThnR may interact with itself when bound to each site via the formation of a DNA loop, as evidenced by the distortion of the DNA between the primary binding sites and the elimination of the higher-order complexes following the introduction of a half-turn of the DNA helix between the primary binding sites. Transcription from each promoter is not fully independent since mutations in each binding site affected transcription from both promoters. Based on these results, we propose a model of transcription activation that involves the formation of a complex structure by interactions between ThnR molecules bound to distant binding sites and favours transcription from one promoter to the detriment of the other.

NtrC-dependent regulatory network for nitrogen assimilation in Pseudomonas putida

Pseudomonas putida KT2440 is a model strain for studying bacterial biodegradation processes. However, very little is known about nitrogen regulation in this strain. Here, we show that the nitrogen regulatory NtrC proteins from P. putida and Escherichia coli are functionally equivalent and that substitutions leading to partially active forms of enterobacterial NtrC provoke the same phenotypes in P. putida NtrC. P. putida has only a single P(II)-like protein, encoded by glnK, whose expression is nitrogen regulated. Two contiguous NtrC binding sites located upstream of the sigma(N)-dependent glnK promoter have been identified by footprinting analysis. In vitro experiments with purified proteins demonstrated that glnK transcription was directly activated by NtrC and that open complex formation at this promoter required integration host factor. Transcription of genes orthologous to enterobacterial codB, dppA, and ureD genes, whose transcription is dependent on sigma(70) and which are activated by Nac in E. coli, has also been analyzed for P. putida. Whereas dppA does not appear to be regulated by nitrogen via NtrC, the codB and ureD genes have sigma(N)-dependent promoters and their nitrogen regulation was exerted directly by NtrC, thus avoiding the need for Nac, which is missing in this bacterial species. Based upon these results, we propose a simplified nitrogen regulatory network in P. putida (compared to that in enterobacteria), which involves an indirect-feedback autoregulation of glnK using NtrC as an intermediary.

Activation and repression of a sigmaN-dependent promoter naturally lacking upstream activation sequences

The Pseudomonas sp. strain ADP protein AtzR is a LysR-type transcriptional regulator required for activation of the atzDEF operon in response to nitrogen limitation and cyanuric acid. Transcription of atzR is directed by the sigma(N)-dependent promoter PatzR, activated by NtrC and repressed by AtzR. Here we use in vivo and in vitro approaches to address the mechanisms of PatzR activation and repression. Activation by NtrC did not require any promoter sequences other than the sigma(N) recognition motif both in vivo and in vitro, suggesting that NtrC activates PatzR in an upstream activation sequences-independent fashion. Regarding AtzR-dependent autorepression, our in vitro transcription experiments show that the concentration of AtzR required for repression of the PatzR promoter in vitro correlates with AtzR affinity for its binding site. In addition, AtzR prevents transcription from PatzR when added to a preformed E-sigma(N)-PatzR closed complex, but isomerization to an open complex prevents repression. Gel mobility shift and DNase I footprint assays indicate that DNA-bound AtzR and E-sigma(N) are mutually exclusive. Taken together, these results strongly support the notion that AtzR represses transcription from PatzR by competing with E-sigma(N) for their overlapping binding sites. There are no previous reports of a similar mechanism for repression of sigma(N)-dependent transcription.

NifL, an antagonistic regulator of NifA interacting with NifA

Nitrogen fixation (nif) genes of diazotrophic enteric bacteria,Enterobacter cloacae orKlebsiella pneumoniae, are regulated bynif LA operon, in which thenif A product, NifA positively regulatesnif gene transcription, whereas the nifL product NifL represses it under oxygen or in excess of fixed nitrogen. Two-hybrid system was used to detect the possible interaction between NifA and NifL. The preliminary results illustrate that NifL does interact with NifA. The interaction between NifL and NtrC has also been shown.

Requirements for translation re-initiation in Escherichia coli: roles of initiator tRNA and initiation factors IF2 and IF3

Despite its importance in post-transcriptional regulation of polycistronic operons in Escherichia coli, little is known about the mechanism of translation re-initiation, which occurs when the same ribosome used to translate an upstream open reading frame (ORF) also translates a downstream ORF. To investigate translation re-initiation in Escherichia coli, we constructed a di-cistronic reporter in which a firefly luciferase gene was linked to a chloramphenicol acetyltransferase gene using a segment of the translationally coupled geneV–geneVII intercistronic region from M13 phage. With this reporter and mutant initiator tRNAs, we show that two of the unique properties of E. coli initiator tRNA – formylation of the amino acid attached to the tRNA and binding of the tRNA to the ribosomal P-site – are as important for re-initiation as for de novo initiation. Overexpression of IF2 or increasing the affinity of mutant initiator tRNA for IF2 enhanced re-initiation efficiency, suggesting that IF2 is required for efficient re-initiation. In contrast, overexpression of IF3 led to a marked decrease in re-initiation efficiency, suggesting that a 30S ribosome and not a 70S ribosome is used for translation re-initiation. Strikingly, overexpression of IF3 also blocked E. coli from acting as a host for propagation of M13 phage.

The LysR-type regulator AtzR binding site: DNA sequences involved in activation, repression and cyanuric acid-dependent repositioning

The LysR-type transcriptional regulator (LTTR) AtzR of Pseudomonas sp. strain ADP activates the cyanuric acid-utilization atzDEF operon in response to low nitrogen availability and the presence of cyanuric acid. AtzR also represses expression of its own gene, atzR, transcribed divergently from atzDEF. Here we identify and functionally characterize the cis-acting sequences at the atzR-atzDEF divergent promoter region required for AtzR-dependent regulation. AtzR binds a single site overlapping both the PatzR and PatzDEF promoters and induces a DNA bend immediately upstream from PatzDEF. Interaction of AtzR with the inducer cyanuric acid shortens the protein-DNA interaction region and relaxes the DNA bend. The AtzR binding site contains a strong binding determinant, the repression binding site (RBS), centred at position -65 relative to the atzDEF transcriptional start, containing the LTTR binding consensus motif. Integrity of the RBS is essential for high-affinity AtzR binding, activation and autorepression. A second, weaker binding determinant, the activation binding site (ABS), is present between the RBS and PatzDEF. Deletion of the ABS only provokes a modest decrease in AtzR affinity for the promoter region in vitro, but abolishes repression of PatzR in vivo. Involvement of the ABS in autorepression has not been previously reported.

Stable long-term indigo production by overexpression of dioxygenase genes using a chromosomal integrated cascade expression circuit

In our laboratory we have analyzed different factors to maximize the yield in heterologous protein expression for long-term cultivation, by combination of an efficient cascade expression system and stable integration in the bacterial chromosome. In this work, we have explored this system for the production of indigo dye as a model for biotechnological production, by expressing in Escherichia coli the thnA1A2A3A4 genes from Sphingomonas macrogolitabida strain TFA, which encode the components of a tetralin dioxygenase activity. We compared Ptac, and the Pm-based cascade expression circuit in a multicopy plasmid and stably integrated into the bacterial chromosome. Plasmid-based expression systems resulted in instability of indigo production when serially diluted batch experiments were performed without a selective pressure. This problem was solved by integrating the expression module in the chromosome. Despite the gene dosage reduction, the synergic effect of the cascade expression system produced comparable expression to the dioxygenase activity in the plasmid configuration but could be stably maintained for at least 5 days. Here, we show that the cascade amplification circuit integrated in the chromosome could be an excellent system for tight control and stable production of recombinant products.

Regulation of tetralin biodegradation and identification of genes essential for expression of thn operons

The tetralin biodegradation genes of Sphingomonas macrogolitabida strain TFA are clustered in two closely linked and divergent operons. To analyze expression of both operons under different growth conditions, transcriptional and translational gene fusions of the first genes of each operon to lacZ have been constructed in plasmids unable to replicate in Sphingomonas and integrated by recombination into the genome of strain TFA. Expression analysis indicated that the transcription of both genes is induced in similar ways by the presence of tetralin. Gene expression in both operons is also subjected to overimposed catabolic repression. Two additional genes named thnR and thnY have been identified downstream of thnCA3A4 genes. ThnR is similar to LysR-type regulators, and mutational analysis indicated that ThnR is strictly required for expression of the thn operons. Unlike other LysR-type regulators, ThnR does not repress its own synthesis. In fact, ThnR activates its own expression, since thnR is cotranscribed with the thnCA3A4 genes. ThnY is similar to the ferredoxin reductase components of dioxygenase systems and shows the fer2 domain, binding a Cys4[2Fe-2S] iron sulfur center, and the FAD-binding domain, common to those reductases. However, it lacks the NAD-binding domain. Intriguingly, ThnY has a regulatory role, since it is also strictly required for expression of the thn operons. Given the similarity of ThnY to reductases and the possibility of its being present in the two redox states, it is tempting to speculate that ThnY is a regulatory component connecting expression of the thn operons to the physiological status of the cell.

Site-directed mutagenesis of an extradiol dioxygenase involved in tetralin biodegradation identifies residues important for activity or substrate specificity

The sequence of the extradiol dioxygenase ThnC, involved in tetralin biodegradation, was aligned with other extradiol dioxygenases involved in biodegradation of polycyclic compounds, and a three-dimensional model of ThnC, based on the structure of the previously crystallized 2,3-dihydroxybiphenyl dioxygenase from Burkholderia fungorum LB400, was built. In order to assess the functional importance of some non-active-site residues whose relevance could not be established by structural information, a number of positions surrounding the substrate-binding site were mutated in ThnC. Ten mutant proteins were purified and their activity towards 1,2-dihydroxytetralin, 1,2-dihydroxynaphthalene and 2,3-dihydroxybiphenyl was characterized. N213H, Q198H, G206M, A282R and A282G mutants increased k(cat)/K(m) at least twofold using 1,2-dihydroxytetralin as the substrate, thus showing that activity of ThnC is not maximized for this substrate. N213H and Q198H mutants increased k(cat)/K(m) using any of the substrates tested, thus showing the relevance for activity of these two histidines, which are highly conserved in dihydroxybiphenyl dioxygenases, but not present in dihydroxynaphthalene dioxygenases. Different substitutions in position 282 had different effects on general activity or substrate specificity, thus showing the functional importance of the most C-terminal beta-sheet of the protein. A251M and G206M mutants showed increased activity specifically for a particular substrate. N213H, G206M, A282R, A282G and Y177I substitutions resulted in enzymes more tolerant to acidic pH, the most striking effect being observed in mutant Y177I, which showed maximal activity at pH 5.5. In addition, Q198D and V175D mutants, which had altered K(m), also showed altered sensitivity to substrate inhibition, thus indicating that inhibition is exerted through the same binding site. This mutational analysis, therefore, identified conserved residues important for activity or substrate specificity, and also shed some light on the mechanism of substrate inhibition exhibited by extradiol dioxygenases.

Mutant forms of the Azotobacter vinelandii transcriptional activator NifA resistant to inhibition by the NifL regulatory protein

The Azotobacter vinelandii sigma(54)-dependent transcriptional activator protein NifA is regulated by the NifL protein in response to redox, carbon, and nitrogen status. Under conditions inappropriate for nitrogen fixation, NifL inhibits transcription activation by NifA through the formation of the NifL-NifA protein complex. NifL inhibits the ATPase activity of the central AAA+ domain of NifA required to drive open complex formation by sigma(54)-RNA polymerase and may also inhibit the activator-polymerase interaction. To analyze the mechanism of inhibition in greater detail, we isolated NifA mutants which are resistant to the inhibitory action of NifL. Mutations in both the amino-terminal GAF domain and the catalytic AAA+ domain of NifA were isolated. Several mutants blocked inhibition by NifL in response to both nitrogen and redox status, whereas some of the mutant NifA proteins were apparently able to discriminate between the forms of NifL present under different environmental conditions. One mutant protein, NifA-Y254N, was resistant to NifL under conditions of anaerobic nitrogen excess but was relatively sensitive to NifL under aerobic growth conditions. The properties of the purified mutant protein in vitro were consistent with the in vivo phenotype and indicate that NifA-Y254N is not responsive to the nitrogen signal conveyed by the interaction of NifL with A. vinelandii GlnK but is responsive to the oxidized form of NifL when ADP is present. Our observations suggest that different conformers of NifL may be generated in response to discrete signal transduction events and that both the GAF and AAA+ domains of NifA are involved in the response to NifL.

Roles of positive and negative feedback in biological systems

We discuss the influence of positive and negative feedback on the stability of a system, which is not clear-cut, and involves complex, mathematical problems. We show in particular that positive feedback can have a stabilising effect on some systems. We also point out the role that positive feedback plays in the digital treatment of signals required by cellular signalling, drawing on analogies from electronics, and the role that negative feedback plays in making a system robust against alteration of its parameters. Both positive and negative feedback can be seen as important enhancers of the properties of biological systems.

Identification of a hydratase and a class II aldolase involved in biodegradation of the organic solvent tetralin

Two new genes whose products are involved in biodegradation of the organic solvent tetralin were identified. These genes, designated thnE and thnF, are located downstream of the previously identified thnD gene and code for a hydratase and an aldolase, respectively. A sequence comparison of enzymes similar to ThnE showed the significant similarity of hydratases involved in biodegradation pathways to 4-oxalocrotonate decarboxylases and established four separate groups of related enzymes. Consistent with the sequence information, characterization of the reaction catalyzed by ThnE showed that it hydrated a 10-carbon dicarboxylic acid. The only reaction product detected was the enol tautomer, 2,4-dihydroxydec-2-ene-1,10-dioic acid. The aldolase ThnF showed significant similarity to aldolases involved in different catabolic pathways whose substrates are dihydroxylated dicarboxylic acids and which yield pyruvate and a semialdehyde. The reaction products of the aldol cleavage reaction catalyzed by ThnF were identified as pyruvate and the seven-carbon acid pimelic semialdehyde. ThnF and similar aldolases showed conservation of the active site residues identified by the crystal structure of 2-dehydro-3-deoxy-galactarate aldolase, a class II aldolase with a novel reaction mechanism, suggesting that these similar enzymes are class II aldolases. In contrast, ThnF did not show similarity to 4-hydroxy-2-oxovalerate aldolases of other biodegradation pathways, which are significantly larger and apparently are class I aldolases.

Improvement of recombinant protein yield by a combination of transcriptional amplification and stabilization of gene expression

We explored the use of a cascade circuit for heterologous gene expression that consists of a regulatory module with a salicylate-inducible system that controls the expression of a second regulator, xylS2, whose product is activated by common inducers. Activation and increasing the concentration of the second regulator synergistically induced heterologous genes downstream of the Pm promoter in the expression module. This module can be placed in multicopy vectors or in the chromosome of a host strain by means of minitransposons. Using reporter genes, we evaluated gene regulation capacity and gross production of the system with different configurations. The highest yield was obtained when the expression module was in a multicopy plasmid after a 6-h induction. However, expression modules in plasmids showed low stability after induction even with selective pressure. The chromosomal configuration had the lowest basal levels and induced levels comparable to those of plasmid configurations, resulting in accumulation of more than 10% of the total protein. Unlike the configurations in plasmids, the yield was maintained for at least 3 days even without selective pressure. In conclusion, the cascade system in the chromosome configuration is more efficient for long-term fermentation because of the great stability of the overexpressing phenotype in spite of the high levels of expression.

Identification of a NifL-like protein in a diazotroph of the beta-subgroup of the Proteobacteria, Azoarcus sp. strain BH72

NifA, the transcriptional activator of nitrogenase (nif) genes, has up to now been described to be regulated in its activity via the sensor NifL only for members of the gamma-subgroup of the PROTEOBACTERIA: This paper reports a functionally similar NifL-like protein outside this group in Azoarcus sp. strain BH72, a diazotrophic grass endophyte belonging to the beta-subgroup of the PROTEOBACTERIA: Its structural genes for nitrogenase (nifHDK) are regulated in response to combined nitrogen and O(2) and expressed endophytically inside rice roots. In order to characterize nitrogen-regulatory genes, an Azoarcus sp. BH72 genomic library was used to select cosmids that complemented a nifA mutation in Azotobacter vinelandii. Sequence analysis of the 3.4 kb genomic region complementing nifA showed two ORFs with sequence identities of 44% to NifL and 61% to NifA of Azotobacter vinelandii. According to Northern blot and reverse transcriptase PCR analysis, the nifLA transcript was more abundant at low combined nitrogen and O(2) levels, results which were corroborated by GUS (beta-glucuronidase) assays using a transcriptional nifL::gusA fusion. N(2) fixation was abolished in a NifLA(-) and a NifA(-) mutant, wild-type fixation being restored by nifLA in trans. The NifLA(-) mutant also failed to activate nifH::gus expression, indicating that NifA is the obligate transcriptional activator for nifHDK. A nifL mutant was diazotrophic and did not show repression of nifH::gusA by ammonium or O(2), suggesting that NifL of Azoarcus sp. strain BH72 has a similar role in inactivating NifA in response to O(2) and combined nitrogen as NifL in bacteria of the gamma-PROTEOBACTERIA:

Molecular characterization of an operon, cueAR, encoding a putative P1-type ATPase and a MerR-type regulatory protein involved in copper homeostasis in Pseudomonas putida

The authors have characterized a chromosomally localized two-gene operon, cueAR, which encodes a putative P1-type ATPase, CueA, and a MerR-type metalloregulatory protein, CueR, in Pseudomonas putida PNL-MK25. Disruption of cueAR by the insertion of mini-Tn5::gfp into the wild-type strain led to a mutant strain with a sixfold reduction in its tolerance to copper; however, the tolerance of this mutant strain to the other seven related transition metals tested was not affected. The sensitivity of the mutant strain was attributed to a higher level of accumulation of intracellular copper, suggesting the involvement of CueA in copper export. Insertion of the cloned cueAR operon into the copper-sensitive mutant strain fully restored its tolerance to copper. cueA::gfp expression studies confirmed that the cueAR operon was transcriptionally regulated by copper and CueR. Studies done on the mutant strain complemented with cueR and cueA revealed partial functional redundancy of cueA and cueR, respectively, in copper tolerance. Thus, the results of this study clearly suggest the involvement of cueAR in copper homeostasis in P. putida.

Role of GlnK in NifL-mediated regulation of NifA activity in Azotobacter vinelandii

In several diazotrophic species of Proteobacteria, P(II) signal transduction proteins have been implicated in the regulation of nitrogen fixation in response to NH(4)(+) by several mechanisms. In Azotobacter vinelandii, expression of nifA, encoding the nif-specific activator, is constitutive, and thus, regulation of NifA activity by the flavoprotein NifL appears to be the primary level of nitrogen control. In vitro and genetic evidence suggests that the nitrogen response involves the P(II)-like GlnK protein and GlnD (uridylyltransferase/uridylyl-removing enzyme), which reversibly uridylylates GlnK in response to nitrogen limitation. Here, the roles of GlnK and GlnK-UMP in A. vinelandii were studied to determine whether the Nif (-) phenotype of glnD strains was due to an inability to modify GlnK, an effort previously hampered because glnK is an essential gene in this organism. A glnKY51F mutation, encoding an unuridylylatable form of the protein, was stable only in a strain in which glutamine synthetase activity is not inhibited by NH(4)(+), suggesting that GlnK-UMP is required to signal adenylyltransferase/adenylyl-removing enzyme-mediated deadenylylation. glnKY51F strains were significantly impaired for diazotrophic growth and expression of a nifH-lacZ fusion. NifL interacted with GlnK and GlnKY51F in a yeast two-hybrid system. Together, these data are consistent with those obtained from in vitro experiments (Little et al., EMBO J., 19:6041-6050, 2000) and support a model for regulation of NifA activity in which unmodified GlnK stimulates NifL inhibition and uridylylation of GlnK in response to nitrogen limitation prevents this function. This model is distinct from one proposed for the related bacterium Klebsiella pneumoniae, in which unmodified GlnK relieves NifL inhibition instead of stimulating it.

Interaction of the antiactivator FleN with the transcriptional activator FleQ regulates flagellar number in Pseudomonas aeruginosa

Flagellar number in Pseudomonas aeruginosa is controlled by FleN, a putative ATP/GTP binding protein. Disruption of fleN results in multiflagellation of the otherwise monoflagellate strains PAK and PAO1 and is associated with a chemotactic defect. We propose that flagellar number is maintained by the antiactivator FleN, which downregulates flagellar genes by binding to their transcriptional activator, FleQ, an enhancer binding protein belonging to the NifA subfamily. In this report we demonstrate direct interaction of FleN and FleQ in the yeast two-hybrid system. Mutagenesis of the putative ATP/GTP binding motif in FleN(24K-->Q) and truncation of FleN at either the N or C terminus abrogates this interaction. FleN does not inhibit the DNA binding ability of FleQ in vitro, thus indicating that it probably utilizes another mechanism(s) to serve as a FleQ antiactivator.

P(II) signal transduction proteins, pivotal players in microbial nitrogen control

The P(II) family of signal transduction proteins are among the most widely distributed signal proteins in the bacterial world. First identified in 1969 as a component of the glutamine synthetase regulatory apparatus, P(II) proteins have since been recognized as playing a pivotal role in control of prokaryotic nitrogen metabolism. More recently, members of the family have been found in higher plants, where they also potentially play a role in nitrogen control. The P(II) proteins can function in the regulation of both gene transcription, by modulating the activity of regulatory proteins, and the catalytic activity of enzymes involved in nitrogen metabolism. There is also emerging evidence that they may regulate the activity of proteins required for transport of nitrogen compounds into the cell. In this review we discuss the history of the P(II) proteins, their structures and biochemistry, and their distribution and functions in prokaryotes. We survey data emerging from bacterial genome sequences and consider other likely or potential targets for control by P(II) proteins.

Protein-protein interactions in the complex between the enhancer binding protein NIFA and the sensor NIFL from Azotobacter vinelandii

The enhancer binding protein NIFA and the sensor protein NIFL from Azotobacter vinelandii comprise an atypical two-component regulatory system in which signal transduction occurs via complex formation between the two proteins rather than by the phosphotransfer mechanism, which is characteristic of orthodox systems. The inhibitory activity of NIFL towards NIFA is stimulated by ADP binding to the C-terminal domain of NIFL, which bears significant homology to the histidine protein kinase transmitter domains. Adenosine nucleotides, particularly MgADP, also stimulate complex formation between NIFL and NIFA in vitro, allowing isolation of the complex by cochromatography. Using limited proteolysis of the purified proteins, we show here that changes in protease sensitivity of the Q linker regions of both NIFA and NIFL occurred when the complex was formed in the presence of MgADP. The N-terminal domain of NIFA adjacent to the Q linker was also protected by NIFL. Experiments with truncated versions of NIFA demonstrate that the central domain of NIFA is sufficient to cause protection of the Q linker of NIFL, although in this case, stable protein complexes are not detectable by cochromatography.

Signal transduction to the Azotobacter vinelandii NIFL-NIFA regulatory system is influenced directly by interaction with 2-oxoglutarate and the PII regulatory protein

PII-like signal transduction proteins, which respond to the nitrogen status via covalent modification and signal the carbon status through the binding of 2-oxoglutarate, have been implicated in the regulation of nitrogen fixation in several diazotrophs. The NIFL-NIFA two-component regulatory system, which integrates metabolic signals to fine-tune regulation of nitrogenase synthesis in Azotobacter vinelandii, is a potential target for PII-mediated signal transduction. Here we demonstrate that the inhibitory activity of the A.vinelandii NIFL protein is stimulated by interaction with the non-uridylylated form of PII-like regulatory proteins. We also observe that the NIFL-NIFA system is directly responsive to 2-oxoglutarate. We propose that the PII protein signals the nitrogen status by interaction with the NIFL-NIFA system under conditions of nitrogen excess, and that the inhibitory activity of NIFL is relieved by elevated levels of 2-oxoglutarate when PII is uridylylated under conditions of nitrogen limitation. Our observations suggest a model for signal transduction to the NIFL-NIFA system in response to carbon and nitrogen status which is clearly distinct from that suggested from studies on other diazotrophs.

Identification of a serine hydrolase which cleaves the alicyclic ring of tetralin

A gene designated thnD, which is required for biodegradation of the organic solvent tetralin by Sphingomonas macrogoltabidus strain TFA, has been identified. Sequence comparison analysis indicated that thnD codes for a carbon-carbon bond serine hydrolase showing highest similarity to hydrolases involved in biodegradation of biphenyl. An insertion mutant defective in ThnD accumulates the ring fission product which results from the extradiol cleavage of the aromatic ring of dihydroxytetralin. The gene product has been purified and characterized. ThnD is an octameric thermostable enzyme with an optimum reaction temperature at 65 degrees C. ThnD efficiently hydrolyzes the ring fission intermediate of the tetralin pathway and also 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid, the ring fission product of the biphenyl meta-cleavage pathway. However, it is not active towards the equivalent intermediates of meta-cleavage pathways of monoaromatic compounds which have small substituents in C-6. When ThnD hydrolyzes the intermediate in the tetralin pathway, it cleaves a C-C bond comprised within the alicyclic ring of tetralin instead of cleaving a linear C-C bond, as all other known hydrolases of meta-cleavage pathways do. The significance of this activity of ThnD for the requirement of other activities to mineralize tetralin is discussed.

Identification of an extradiol dioxygenase involved in tetralin biodegradation: gene sequence analysis and purification and characterization of the gene product

A genomic region involved in tetralin biodegradation was recently identified in Sphingomonas strain TFA. We have cloned and sequenced from this region a gene designated thnC, which codes for an extradiol dioxygenase required for tetralin utilization. Comparison to similar sequences allowed us to define a subfamily of 1, 2-dihydroxynaphthalene extradiol dioxygenases, which comprises two clearly different groups, and to show that ThnC clusters within group 2 of this subfamily. 1,2-Dihydroxy-5,6,7, 8-tetrahydronaphthalene was found to be the metabolite accumulated by a thnC insertion mutant. The ring cleavage product of this metabolite exhibited behavior typical of a hydroxymuconic semialdehyde toward pH-dependent changes and derivatization with ammonium to give a quinoline derivative. The gene product has been purified, and its biochemical properties have been studied. The enzyme is a decamer which requires Fe(II) for activity and shows high activity toward its substrate (V(max), 40.5 U mg(-1); K(m), 18. 6 microM). The enzyme shows even higher activity with 1, 2-dihydroxynaphthalene and also significant activity toward 1, 2-dihydroxybiphenyl or methylated catechols. The broad substrate specificity of ThnC is consistent with that exhibited by other extradiol dioxygenases of the same group within the subfamily of 1, 2-dihydroxynaphthalene dioxygenases.

Studies on the roles of GlnK and GlnB in regulating Klebsiella pneumoniae NifL-dependent nitrogen control

In Klebsiella pneumoniae, nitrogen fixation (nif) genes are regulated in response to fixed nitrogen and oxygen. The activity of the nif-specific transcriptional activator NifA is modulated by NifL, which mediates both oxygen and nitrogen control. The signal transduction protein GlnK is required to relieve the inhibitory effect of NifL on NifA that occurs when the intracellular N status is high and in a wild-type cell, the action of GlnK cannot be substituted by the structurally related protein PII. We have studied the modulation of NifA activity by NifL in an heterologous system in which the host organism is Escherichia coli. Using a DeltaglnB, DeltaglnK mutant, we have shown that the modulation of NifA activity by NifL is dependent on the concentration of GlnK in the cell and that when overproduced, PII can substitute for GlnK. Furthermore, our data suggest that PII can counteract the positive action of GlnK in relieving NifL-dependent inhibition of NifA activity. This negative effect of PII may be physiologically important in establishing repression of nif gene expression when the intracellular nitrogen status rises.

Mechanisms for redox control of gene expression

This review discusses various mechanisms that regulatory proteins use to control gene expression in response to alterations in redox. The transcription factor SoxR contains stable [2Fe-2S] centers that promote transcription activation when oxidized. FNR contains [4Fe-4S] centers that disassemble under oxidizing conditions, which affects DNA-binding activity. FixL is a histidine sensor kinase that utilizes heme as a cofactor to bind oxygen, which affects its autophosphorylation activity. NifL is a flavoprotein that contains FAD as a redox responsive cofactor. Under oxidizing conditions, NifL binds and inactivates NifA, the transcriptional activator of the nitrogen fixation genes. OxyR is a transcription factor that responds to redox by breaking or forming disulfide bonds that affect its DNA-binding activity. The ability of the histidine sensor kinase ArcB to promote phosphorylation of the response regulator ArcA is affected by multiple factors such as anaerobic metabolites and the redox state of the membrane. The global regulator of anaerobic gene expression in alpha-purple proteobacteria, RegB, appears to directly monitor respiratory activity of cytochrome oxidase. The aerobic repressor of photopigment synthesis, CrtJ, seems to contain a redox responsive cysteine. Finally, oxygen-sensitive rhizobial NifA proteins presumably bind a metal cofactor that senses redox. The functional variability of these regulatory proteins demonstrates that prokaryotes apply many different mechanisms to sense and respond to alterations in redox.

Repression of IS200 transposase synthesis by RNA secondary structures

The IS 200 transposase, a 16 kDa polypeptide encoded by the single open reading frame (ORF) of the insertion element, has been identified using an expression system based on T7 RNA polymerase. In wild-type IS 200, two sets of internal inverted repeats that generate RNA secondary structures provide two independent mechanisms for repression of transposase synthesis. The inverted repeat located near the left end of IS 200 is a transcriptional terminator that terminates read-through transcripts before they reach the IS 200 ORF. The terminator is functional in both directions and may terminate >80% of transcripts. Another control operates at the translational level: transposase synthesis is inhibited by occlusion of the ribosome-binding site (RBS) of the IS 200 ORF. The RBS (5'-AGGGG-3') is occluded by formation of a mRNA stem-loop structure whose 3' end is located only 3 nt upstream of the start codon. This mechanism reduces transposase synthesis approximately 10-fold. Primer extension experiments with AMV reverse transcriptase have provided evidence that this stem-loop RNA structure is actually formed. Tight repression of transposase synthesis, achieved through synergistic mechanisms of negative control, may explain the unusually low transposition frequency of IS 200.

Isolation and properties of the complex between the enhancer binding protein NIFA and the sensor NIFL

In Azotobacter vinelandii, activation of nif gene expression by the transcriptional regulatory enhancer binding protein NIFA is controlled by the sensor protein NIFL in response to changes in levels of oxygen and fixed nitrogen in vivo. NIFL is a novel redox-sensing flavoprotein which is also responsive to adenosine nucleotides in vitro. Inhibition of NIFA activity by NIFL requires stoichiometric amounts of the two proteins, implying that the mechanism of inhibition is by direct protein-protein interaction rather than by catalytic modification of the NIFA protein. The formation of the inhibitory complex between NIFL and NIFA may be regulated by the intracellular ATP/ADP ratio. We show that adenosine nucleotides promote complex formation between purified NIFA and NIFL in vitro, allowing isolation of the NIFL-NIFA complex. The complex can also be isolated from cell extracts containing coexpressed NIFL and NIFA in the presence of MgADP. Removal of the nucleotide causes dissociation of the complex. Experiments with truncated proteins demonstrate that the amino-terminal domain of NIFA and the C-terminal region of NIFL potentiate the ADP-dependent stimulation of NIFL-NIFA complex formation.

Genetic analysis of biodegradation of tetralin by a Sphingomonas strain

A strain designated TFA which very efficiently utilizes tetralin has been isolated from the Rhine river. The strain has been identified as Sphingomonas macrogoltabidus, based on 16S rDNA sequence similarity. Genetic analysis of tetralin biodegradation has been performed by insertion mutagenesis and by physical analysis and analysis of complementation between the mutants. The genes involved in tetralin utilization are clustered in a region of 9 kb, comprising at least five genes grouped in two divergently transcribed operons.

Physiological role for the GlnK protein of enteric bacteria: relief of NifL inhibition under nitrogen-limiting conditions

In Klebsiella pneumoniae, NifA-dependent transcription of nitrogen fixation (nif) genes is inhibited by a flavoprotein, NifL, in the presence of molecular oxygen and/or combined nitrogen. We recently demonstrated that the general nitrogen regulator NtrC is required to relieve NifL inhibition under nitrogen (N)-limiting conditions. We provide evidence that the sole basis for the NtrC requirement is its role as an activator of transcription for glnK, which encodes a PII-like allosteric effector. Relief of NifL inhibition is a unique physiological function for GlnK in that the structurally related GlnB protein of enteric bacteria-apparently a paralogue of GlnK-cannot substitute. Unexpectedly, although covalent modification of GlnK by uridylylation normally occurs under N-limiting conditions, several lines of evidence indicate that uridylylation is not required for relief of NifL inhibition. When GlnK was synthesized constitutively from non-NtrC-dependent promoters, it was able to relieve NifL inhibition in the absence of uridylyltransferase, the product of the glnD gene, and under N excess conditions. Moreover, an altered form of GlnK, GlnKY51N, which cannot be uridylylated due to the absence of the requisite tyrosine, was still able to relieve NifL inhibition.

Mechanism of translational coupling in the nifLA operon of Klebsiella pneumoniae

The nifLA operon of Klebsiella pneumoniae encodes the sensor-activator pair involved in the regulation of other nif genes. Balanced synthesis of both proteins, which is required for correct regulation, is achieved by coupling translation of nifA to that of nifL. The mechanism of translational coupling at the nifLA operon was analysed using a specialized ribosome system, and the effect of substituting the natural Shine-Dalgarno of nifL or nifA for specialized Shine-Dalgarno sequences was determined. Our results indicate that translational coupling occurs in this operon by a reinitiation mechanism. Additionally, reinitiation at the nifA can happen even in the absence of good Shine-Dalgarno recognition by the reinitiating ribosome, although its efficiency is lower. The effect of a putative translational enhancer sequence (downstream box) on translational coupling efficiency was also determined. Mutations that reduce the homology of the putative downstream box to the consensus had only a minor effect on nifA translation by wild-type ribosomes. However, they had a significant effect on nifA translation by specialized ribosomes, suggesting that recognition of the downstream box may compensate inefficient ribosomal interactions with the Shine-Dalgarno sequence.

The redox- and fixed nitrogen-responsive regulatory protein NIFL from Azotobacter vinelandii comprises discrete flavin and nucleotide-binding domains

Azotobacter vinelandii NIFL is a nitrogen fixation-specific regulatory flavoprotein that modulates the activity of the transcriptional activator NIFA in response to oxygen and fixed nitrogen in vivo. NIFL is also responsive to ADP in vitro. Limited proteolysis of NIFL indicates that it comprises a relatively stable N-terminal domain and a C-terminal domain that is protected from trypsin digestion in the presence of adenosine nucleotides. ATP protects the protein from cleavage in the vicinity of potential nucleotide-binding sites in the C-terminus, whereas ADP protects the entire C-terminal domain. NIFL has an apparent Kd of 130 microM for ATP and 16 microM for ADP. The purified N-terminal domain has an identical UV/visible absorption spectrum to the wild-type protein and is reduced by sodium dithionite, demonstrating that it is a flavin-binding domain. The isolated N-terminal domain does not inhibit NIFA activity. A subdomain fragment containing 160 residues of the C-terminal region, including the nucleotide-binding sites, is also not competent to inhibit NIFA. Removal of the first 146 residues of NIFL, which includes a conserved S-motif (PAS-like domain), found in a large family of sensory proteins from eubacteria, archea and eukarya eliminates the redox response. However, this truncated protein remains competent to inhibit NIFA activity in response to ADP in vitro and to the level of fixed nitrogen in vivo. The redox and nitrogen-sensing functions of A. vinelandii NIFL are therefore separable and are discrete functions of the protein.

NtrC is required for control of Klebsiella pneumoniae NifL activity

In response to molecular oxygen and/or fixed nitrogen, the product of the Klebsiella pneumoniae nitrogen fixation L (nifL) gene inhibits NifA-mediated transcriptional activation. Nitrogen regulation of NifL function occurs at two levels: transcription of the nifLA operon is regulated by the general Ntr system, and the activity of NifL is controlled by an unknown mechanism. We have studied the regulation of NifL activity in Escherichia coli and Salmonella typhimurium by monitoring its inhibition of NifA-mediated expression of a K. pneumoniae phi(nifH'-'lacZ) fusion. The activity of the NifL protein transcribed from the tac promoter is regulated well in response to changes of oxygen and/or nitrogen status, indicating that no nif- or K. pneumoniae-specific product is required. Unexpectedly, strains carrying ntrC (glnG) null alleles failed to release NifL inhibition, despite the fact that synthesis of NifL was no longer under Ntr control. Additional evidence indicated that it is indeed the transcriptional activation capacity of NtrC, rather than its repression capacity, that is needed, and hence it is a plausible hypothesis that NtrC activates transcription of a gene(s) whose product(s) in turn functions to relieve NifL inhibition under nitrogen-limiting conditions.